<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>js_外部js</title>
    <style>
        #btn  {
            width: 80px;
            height: 30px;
            background-color: #05B2D2;
        }
    </style>
</head>
<body>
<div id="btn"></div>
<script src="./main.js"></script>
</body>
</html>